Processes and plants for the production of hot-rolled steel strip which use a Steckel rolling mill with one or more reversing stands normally fed with slabs with a thickness from 150 to 250 mm.
Such plants typically provide, a slab casting machine, shearing means, a heating furnace to restore, maintain or homogenize the temperature of the cast slab so that it is suitable for subsequent rolling, a high-pressure water de-scaler, a Steckel reversing rolling train with one or two stands, a laminar cooling system and a winding unit to wind the strip into rolls of a predefined weight.
It is also known that rolling plants with a Steckel reversing rolling train with one or more stands, which use a slab with a thickness from 150 to 250 mm or more have limitations in terms of the minimum thickness obtainable and in the quality, both of size and surface, of the final strip.
Indeed, because of the great thickness of the starting slab, the large number of rolling passes through the stand/stands and consequently of the long downtimes of inversion, with consequent long total times from beginning to end of rolling, there is a big loss of temperature which makes it impossible to roll thin thicknesses of final product, for example 1.6-1.4 mm or less.
Moreover, there is lack of homogeneity in the temperature along the strip and the formation of scale, which negatively affect the quality of the strip produced.
Finally, the surface quality of the finished product also suffers from the effect of using the work rolls for the numerous passes of the cold leading/tail ends and the consequent rapid deterioration of the surface of the rolls themselves. To reduce this disadvantage, it is necessary to change the work rolls frequently, with consequent stoppages, compromising the factor of use and productivity of the plant.
Another problem found is such production lines is the overall great length of the line, which negatively affects not only the investment costs but also the energy costs of production and maintenance costs.
WO-A-00/10741 describes a rolling method that, in one form of embodiment, provides a continuous casting step, a roughing step, directly downstream of casting, a heating step carried out after roughing and upstream of a finishing rolling step. In another alternative form of embodiment of WO'741, between the roughing step and the heating step a winding/unwinding step is provided. In another alternative form of embodiment of WO'741, the heating step is the rapid type and is provided directly downstream of casting, whereas the roughing step is provided after the rapid heating, very distant from casting. After the roughing step a winding/unwinding step is provided, after a possible further heating step, which makes the method and connected rolling line according to WO'741 more expensive and dimensionally bigger, and finally the finishing rolling,
WO-A-2010/115698 describes a rolling method that only provides a continuous casting step, a roughing step, a rapid heating step after roughing, a step of detecting scale, a pre-cooling step, a de-scaling step and finally a finishing rolling step.
JP-A-59191502 describes a rolling unit provided with a single Steckel type rolling stand, equipped with induction type heating means disposed between the rolls of the rolling stand and the reel furnaces at entrance to and exit from the rolling stand.
Primary purpose of the present invention is to obtain a process and corresponding line for the hot production of steel strip which is extremely compact, with low investment costs, which allows to obtain final thicknesses of 1.4 mm or less.
Another purpose is to obtain a finished product with good quality in terms of less scale impressed, good surface quality and dimensional tolerance even along the length.
Another purpose of the present invention is to perfect a method that allows to reduce to a minimum the number of rolling passes and inversions, and hence to reduce the total rolling time, with consequent greater uniformity/homogeneity of temperature along the strip being rolled and a lesser overall loss of temperature of the strip.
Furthermore, another purpose is to obtain a production line that transforms, in a single continuous cycle, without intermediate storage and recovery of material, and with extremely limited energy consumption, the liquid steel arriving from the steel works and which is able to produce, at competitive costs compared to other, conventional technologies, final product in a range comprised from about 300,000 to about 800,000 tonnes per year.
Another purpose is to increase the factor of use of the production line, increasing the operating life of the work rolls.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.